Noise attenuator arrangement

ABSTRACT

Apparatus for the release of pressurised fluids comprising an opening through which pressurised fluids may pass and an attenuator member extending over said opening. The attenuator member operates to control the flow of said fluid and attenuate sound associated therewith.

This invention relates to an attenuator for reducing noise associatedwith an air bleed system particularly, but not exclusively, for aturbofan aircraft engine.

In an aircraft engine, particularly a large high bypass ratio turbofan,It may be necessary for the best operation of the engine to bleed airpressure from within the compressor. To do this, a servo-controlledvalve can be provided in ducting leading from the compressor casing tothe fan by-pass duct. Normally, there would be an array of several bleedvalves spaced around the axis of the engine. Also, two or more valves orvalve arrays may be provided at different stages of the compressor. Thedifferent valves or valve arrays are opened and dosed, or sometimesmodulated, i.e. set to give a required flow rate between on and off byan engine management system. The valves are controlled by this systemalong with other engine parameters to optimise the operation of theengine for different operating conditions.

The release of high pressure air into the fan by-pass duct can createconsiderable noise and, at least in relation to valves which may open atlow altitude when the aircraft is taxiing, taking off or landing, soundattenuation is required.

Patent application number GB2,132,269A discloses an attenuator for a gasturbine engine air bleed valve. The attenuator receives the plume ofhigh pressure air from a compressor stage of the engine and has a largenumber of small holes which convert the stream to a multiplicity of airjets.

Patent application number EP0354161A3 discloses a muffler plate for arefrigeration system compressor with a few relatively large holesarranged round a check valve.

U.S. Pat. No. 5,906,225 shows a refrigeration system expansion valvewith an elongate attenuator member made of porous material.

According to the invention, there is provided an air bleed system for aturbofan aircraft engine, the system comprising duct means for receivingair from a compressor stage of the engine and directing said air to abypass duct of the engine valve means within the duct means forcontrolling the flow of said air, and a sound attenuator memberextending over the duct means downstream and having aperturesdistributed over the member for passing said air, characterised in thatthe apertures of the attenuator member are arranged for the distributionof the air passing through the attenuator member to be relatively morerestricted in a central region of the member than in another regionoutside said central region.

Said another region is an annular region extending round said centralregion.

Preferably, the valve and attenuator member are constructed for thepressure drop from the upstream to the downstream side of the valvemember to be substantially equal to the pressure drop through theattenuator member.

The attenuator member may be positioned for a high pressure stream ofair from the valve to Impinge upon said central region.

Advantageously, the attenuator member comprises a substantial number ofrelatively small perforations distributed over a peripheral region ofthe attenuator member and no or relatively few such apertures in thecentral region.

Preferably, the attenuator member has apertures differentiallydistributed over the attenuator member and porous material adjacent atleast some of the apertures.

The porosity and/or thickness of the porous material may be different indifferent regions of the attenuator member.

The attenuator member may comprise porous material for defining saidapertures, the porosity and/or thickness of the porous material beingdifferent in different regions of the attenuator member.

The porous material may be porous metallic foam.

The valve may be a bullet valve.

Referring to FIGS. 4 to 6, there is shown a number of alternativelayouts of holes in the cover 29 of the attenuator 16. The size, shapeand positioning of the holes 31, 32, 33 may be changed to suit differentflow-rates of air. A Retimet foam layer 35 may be provided beneath themember 16.

In the embodiment shown in FIG. 7, the cover 29 of the attenuator 16 isomitted and effectively replaced entirely by a self supporting layer ofRetimet foam layer 35.

The metal foam 35 could incorporate different grades of foam; forexample, the centre area 32 could be of a grade more restrictive to flowthan the outer area.

Foams are graded with a number system representing the number ofpores/cells per linear inch; e.g. 80 grade has 80 cells per inch. It isexpected that the range of foams suitable for this application would bein the range 5 to 150 grade, and preferably be in the range 10 to 80grade.

Changes of the grade of the layer of metal foam 35 of FIGS. 3 to 7 couldproduce different noise reduction characteristics. For example differentgrades of foam with different porosity would reduce different noisefrequencies.

A sandwich structure of layers of foam of different grades wouldalternate a wider range of noise frequencies and provide improved noisereduction. Also, by using foams of different thickness one could alsochange the noise reduction properties by changing the flow rate. Thisneeds to be balanced against the desired flow rate from the valve 6 andthe pressure drop across the valve 6.

The layer of metallic foam 35 of FIGS. 3 to 6 may have different zonesof different porosity aligned with selected holes 31, 32, 33 in thecover 29. Similarly, the inserts of metallic foam 35 of FIG. 7 can havea different porosity for different zones. In this way, one canaccommodate different airflow rates or different levels of soundattenuation for different applications and thus provide greaterflexibility in the design of the characteristics of the attenuator.

It is to be understood that the layer 35 of metallic foam shown in FIGS.3 to 6 may be replaced by individual inserts of metallic foam that aresecured in each of the holes 31, 32, 33.

It may be possible to use a mat of metal or other fibres to produce asimilar effect to the metal foam to control flow rate and pressure dropacross the valve 6 to reduce noise. Such a mat could be a woven ornon-woven fibre structure or fabric.

The cover 29 might be required or desirable in some of thoseapplications where a foam metal layer is used, for example, to restrictair flow more in some areas than in other areas. But in otherapplications the cover 29 may not be needed. Where a cover 29 is used itmay be of any support structure such as perforated metal or plasticsmaterial or could simply comprise two mutually orthogonal sets ofparallel wires or wire mesh to retain the foam metal layer 35 in place.

1. An air bleed system for a turbofan aircraft engine, the systemcomprising duct means for receiving air from a compressor stage of theengine and directing said air to a bypass duct of the engine, valvemeans within the duct means for controlling the flow of said air, and asound attenuator member extending over the duct means downstream of thevalve means and having apertures distributed over the member for passingsaid air, characterised in that the apertures of the attenuator memberare arranged for the distribution of the air passing through theattenuator member to be relatively more restricted in a central regionof the member than in another region outside said central region.
 2. Asystem according to claim 1, wherein said another region is an annularregion extending round said central region.
 3. A system according toclaim 1, wherein the valve and attenuator member are constructed for thepressure drop from the upstream to the downstream side of the valvemember to be substantially equal to the pressure drop through theattenuator member.
 4. A system according to claim 1, wherein theattenuator member is positioned for a high pressure stream of air fromthe valve to impinge upon said central region.
 5. A system according toclaim 1, wherein the attenuator member comprises a substantial number ofrelatively small perforations distributed over a peripheral region ofthe attenuator member and no or relatively few such apertures in thecentral region. 6-10. (canceled).
 11. A system according to claim 1,wherein the attenuator member comprises porous material having poresextending through the attenuator member to define said apertures.
 12. Asystem according to claim 11, wherein the porous material is porousmetallic foam.
 13. A system according to claim 1, wherein the attenuatormember has apertures differentially distributed over the attenuatormember and porous material adjacent at least some of the apertures. 14.A system according to claim 13, wherein the porous material is porousmetallic foam.
 15. A system according to claim 13, wherein the porosityof the porous material is different in different regions of theattenuator member.
 16. A system according to claim 13, wherein thethickness of the porous material is different in different regions ofthe attenuator member.
 17. Apparatus according to claim 1, wherein thevalve is a bullet valve.